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Old 13-10-2017, 07:29   #1
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Anchor bridle/snubber

Catamarans tend to have long bridles, often 7m or more.

When paying out rode, do you count the length of the bridle as part of the scope say 4:1, of do you go 4:1 then attach the bridle and let out more rode?

I've been including the length of the bridle part of my scope and usually gone 4:1 or 5:1, but wondering whether that is correct as although the pull from the boat is much lower down than it would be without a bridle, it's pulling on less rode overall.

What do you do? And do your change your practise depending on the depth of the water?
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Old 13-10-2017, 08:00   #2
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Re: Anchor bridle/snubber

Regardless of mono or multihull, it is the total length that matters, because it is the total, plus the height of freeboard and the depth of the water, that determines the angle of the rode and the holding efficiency. Pete
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Old 13-10-2017, 08:13   #3
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Re: Anchor bridle/snubber

If the bridle is Attached to the waterline, I canít the scope as the depth of the water versus the last of the road. Itís a bridal is attached above the waterline I canít scope as the attachment point of the bridal versus the depth of the water. Attaching below the waterline effectively reduces the height of the attached point.
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Old 13-10-2017, 16:00   #4
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Re: Anchor bridle/snubber

As has been already stated, the bridle is part of the rode and should be included. For a theoretical calculation of scope, there are two numbers that matter:
- the length of the rode system (including bridle) to the attachment point on the boat that takes the load (where your bridle attaches - if two, imagine a single point on center line)
- the vertical distance to the bottom from the attachment point (depth of the water + the height of the attachment point above the waterline)

Don't forget to include the height above the water in the calculation as that can be a significant factor, particularly in shallow water with a high freeboard boat.

This is all in theory. In practice we found that while cruising rode length is often constrained by other constraints such as swing room, but it is important to know how to calculate actual scope to make sure you know what your holding is likely to be. We learnt with time to be comfortable with widely different scopes/holding depending on location, weather forecasts and our plans (staying aboard? going ashore for the day? etc).
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Old 14-10-2017, 10:34   #5
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Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
SNIP

What do you do? And do your change your practise depending on the depth of the water?
Another consideration is if you are using all chain or part chain and part line.

I can only speak for my boat but I tend to use a much shorter bridle than you indicate to reduce possible chafe due to where the cleats on my hull are. I also normally use all chain since I have something like 175 feet of chain on my primary. My take is you need more line out that chain. That being said I try to go at least 7:1 with all chain and often will let out more; but always in stages. I let out water depth plus a few feet; drift back, let out a little more and drift back; repeating maybe 3,4, or 5 times, and then back down; then let out even more chain. All of this is usually in 10 feet of water. When I am in deeper water things change a lot; but I always try and anchor in fairly shallow water.
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Old 14-10-2017, 10:49   #6
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Re: Anchor bridle/snubber

What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
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Old 14-10-2017, 11:40   #7
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Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
If your anchor is in 5m and your freeboard is 1m and you let out 21m of chain, your scope is 21/6= 3.5:1, not 4:1.

If you let out 16m of chain and a 5m bridle (assuming this is length to the bow roller. The total length of the bridal on a cat will be greater due to the beam) then there is still 21m out and the scope is the same 3.5:1.

The total weight of chain will be similar in both cases, assuming the chain still leads to the bow roller.
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Old 14-10-2017, 11:47   #8
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Re: Anchor bridle/snubber

Fair enough. I never multiple the freeboard it doesn't make any sense to me. It's always the same height, it doesn't change regardless of the depth of water, so all my calculations are usually done from water level.
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Old 14-10-2017, 11:55   #9
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Re: Anchor bridle/snubber

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Originally Posted by mikedefieslife View Post
Fair enough. I never multiple the freeboard it doesn't make any sense to me.
The holding power of the anchor is related to the angle of the upward pull on the anchor. 1m of freeboard or 1m of extra depth has the same effect on the geometry.

So the freeboard should be added to the depth if you want to calculate the scope correctly.
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Old 14-10-2017, 12:08   #10
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Re: Anchor bridle/snubber

Very true. I suppose that is usually covered in my 'let a bit extra' out which I always seem to do.
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Old 14-10-2017, 12:22   #11
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Re: Anchor bridle/snubber

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Originally Posted by mikedefieslife View Post
Very true. I suppose that is usually covered in my 'let a bit extra' out which I always seem to do.
A bit extra never hurts .

Dont worry about the snubber length just look at the amount of chain out at the bow (assuming there is not an large loop of chain between the bow roller and the snubber conection) divide this by the depth of water where the anchor is located plus the freeboard. This ratio is the scope.

Some people add the maximium rise in tide but this is more correctly termed the minimum expected scope.
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Old 14-10-2017, 12:24   #12
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Re: Anchor bridle/snubber

I use 200 feet of 3/8 inch high test chain when anchoring. One of the most important parts of our anchoring system is a lazy loop of chain that makes the pull of the chain more horizontal along the sea bed. It works really well for us, and we always use the lazy loop. We can anchor on shorter scope, and the additional weight of the lazy loop makes it harder for wind gusts to dislodge the anchor.

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Old 14-10-2017, 12:40   #13
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Re: Anchor bridle/snubber

A "lazy loop" adds a bit of chain catenary and if some chain is dragging on the bottom it reduces sheering.

The drawback is if the snubber breaks, the boat will move back some distance, and develop considerable momentum, placing a high shock load on the anchor.

If using this technique (and it does have merits) I would advise using two independent snubbers. Cats using a bridle often rig this routinely. Two independent snubbers are not often used on a monohulls, but they are a good precaution in extreme conditions.
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Old 16-10-2017, 04:51   #14
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Re: Anchor bridle/snubber

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Originally Posted by mikedefieslife View Post
What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
The catenary effect is always irrelevant when it's most needed, that means whenever there is any significant load on the rode.
I'll give you a numerical example of this:

~10 mm chain, having effective weight of 20 N/m when submerged.
Horizontal load on the rode 10 kN.
Horizontal distance from anchor to the attachment point 50 m.
Vertical distance from anchor to the attachment point 12.636 m.
Straight line distance from anchor to the attachment point 51.572 m.
Distance along the chain (catenary)from anchor to the attachment point 51.592 m.
Added distance due to catenary 0.020 m (2 cm or 20 mm or ~25/32 inch).
Change of angle of rode at the anchor ~2.75 degrees.

For those more interested of the mathematics than the results:
Equation of the rode Y = 500 m * COSH(X/500 m)
End point of rode at the anchor (X =150m Y = 522.669 m) and at the attachment point (X = 100 m Y = 510.033 m) Angle of rode at anchor from horizontal 16,937 degrees. Scope 4.083

If there is only 2 kN of load on the rode, the extension due to catenary would be 0.506 m instead of 0.02 m. That would allow some absorbing of impact loading due to waves and gusts, but still less than a proper snubber. Also the end of rode at anchor would be horizontal, while at the attachment 27.5 degrees from vertical, a significant difference, unlike the 10 kN loaded case of only 2.75 degrees of difference. The vertical distance in the 2 kN load case is 12.76 m instead of 12.64 m , but that doesn't have much effect on the results, just made calcs easier.
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Old 04-11-2017, 06:15   #15
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Re: Anchor bridle/snubber

In Hurricane Wilma both my primary 5/8" nylon and my backup 5/8" nylon snubber snapped, my anchor windless pulled out of its mount, and my chain caught in the hawse pipe only because I was told to tie an overhand knot in the chain itself before shackling it to the hard point - that pulled out of the builder's backing plate.

Had the chain not jammed in the hawse pipe, the boat would have been lost.

While I had two double snubbers - as the previous writer suggested, both halves snapped independently as the splice between the snubber and the anchor chain grabber - a Wichard in this case - held.


I believe in belts and suspenders - but I don't think two snubbers helped my situation.

When the boat, a 43' cat with a 25' beam ran down the all chain rode as the wind veer passed over the boat - the sudden load of fetching up on the anchor snapped the bridles and stretched the chain - the primary anchor - a Bruce 25 KG tripped but the tandem anchor, a Fortress FX-39, held - destroying its flukes and bending the shank in the process.

No ground tackle is perfect, but I remain convinced a plough type anchor with a danforth type anchor shackled to the trip point gives your boat the best chance of surviving a serious storm.

I especially like my setup because the Bruce - or my current boat's Mantus, is easy to deploy and recover in 90% of the conditions - when more is expected, I carry the Fortress and its 6' of chain to the bow, shackle it to the plough type and throw it overboard before I enter the anchroage.

Just deploy chain to the water depth minus about 3 feet so the Fortress drags forward of the boat while you slowly back. Then slowly back and deploy your calculated chain until you get to your bridle point, deploy the remaining chain plus the necessary chain slack to just not touch the bottom and have a beer. It never, ever, failed to hold in my anchoring experience in all kinds of bottoms, with the Fortress having weight added by the horizontal pull from the plough, it is a sure thing - at least as sure as anchoring can be.

My anchoring experience has been better than my mooring experience - twice I've moored in water to deep to inspect and have had moorings fail. Several hurricanes, many squalls and countless thoughtless boat wakes have never had an anchor set as I describe slip. Sleep soundly /Stu
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